As molds used for injection molding resins, there have been mainly used metal molds, and in some cases, for example, for trial production or small-quantity production, there have been used resin molds.
As disclosed in Japanese Patent Laid-Open Publication No. 2001-105438 entitled “Method for Producing Resin Mold”, the latter resin molds are characterized in that a plane facing to a cavity, that is, a product's surface forming plane of the resin mold is made from a resin.
An external appearance plane of a resin product such as a handle cover, a luggage box, a fender, or a sheet bottom plate of a two-wheeled vehicle (motorcycle or scooter type vehicle) has come to be regarded as a decorative design plane, and attempts have been made to give a fine pattern to the decorative design plane of the resin product.
FIG. 20 hereof is a bottom view of a sheet bottom plate of a vehicle. If any decorative design pattern is not given to a sheet bottom plate 100, there arises no problem during running of the vehicle because the sheet bottom plate 100 is not viewed from outside; however, there arises a problem when a sheet is raised up for taking a helmet in or out of a luggage box disposed under the sheet because the sheet bottom plate 100 is exposed and thereby become conspicuous. In particular, for a motorcycle of a type in which a sheet is perfectly removable from a vehicular body, the sheet is removed not only for taking a small-sized luggage in or out of a luggage box but also for cleaning and inspecting the luggage box, and in this case, the sheet bottom plate 100 is in a state viewable from outside until next mounting of the sheet. Accordingly, even for the sheet bottom plate 100, it may be desirable to give a decorative design pattern thereto.
For the reason described above, as shown in FIG. 20, a decorative design pattern 101 is given to a bottom plane of the seat bottom plate 100. The decorative design pattern 101 can be selected from various patterns such as a leather-like pattern, a crimp pattern, and a satin-like pattern, and in the example shown in FIG. 20, it is exemplified by a lattice pattern for convenience of easy depiction thereof. The lattice pattern as the decorative design pattern 101 includes lateral grooves and longitudinal grooves, and in order to enhance the decorative design characteristic, the lateral grooves of the lattice pattern are taken as deep grooves 102 . . . (symbol “. . . ” denotes a plural number, the same applying in the following), and the longitudinal grooves of the lattice pattern are taken as shallow grooves 103 . . . .
FIG. 21 is a sectional view taken on line 21—21 of FIG. 20, showing a state that the deep grooves 102 . . . and the shallow grooves 103 . . . perpendicular thereto are formed in the sheet bottom plate 100. To form a lattice pattern having lateral and longitudinal grooves different in depth from each other as described above, a mold is required to have the following product's surface forming plane.
FIG. 22 is a perspective view of a product's surface forming plane of a mold. To form a lattice pattern on a sheet bottom plate, a reversal pattern composed of projection ribs 107 (height: h1) for forming deep grooves and projecting ribs 108 (height: h2, h2<h1) for forming shallow grooves is required to be engraved in a product's surface forming plane 106 of a mold 105.
If the mold 105 is configured as a metal mold, the above-described reversal pattern can be easily formed on the product's surface forming plate by NC machining, electric charge machining, or etching.
If the mold 105 is configured as a resin mold, however, the above-described reversal pattern cannot be obtained by NC machining, electric charge machining, or etching. To be more specific, the use of NC machining for micromachining the resin mold has a problem that since a resin of the resin mold is melted by cutting heat, it fails to engrave a sharp reversal pattern in the resin mold, and since a resin of the resin mold is an insulator and has a resistance against chemicals, neither electric discharge machining nor etching is used for micromachining the resin mold. Accordingly, a resin mold used as an easy-to-use mold has been not subjected to micromachining.
Resin molds for small-quantity production, however, are required to give patterns to resin products, and therefore, it is required to establish a surface micromachining technology for resin molds.